The U.S. agency alleged in a statement that the incident, from August 2011, involved the faulty installation of a switch on a Boeing 717 that allows crews to test the plane's windshield heating system. The plane was operated by AirTran, which Southwest bought in 2011.

The FAA added in its statement that the Boeing 717 was used on more than 1,100 flights before the switch was fixed.

The agency also said it also proposed a $304,000 civil fine for Great Lakes Aviation , saying that company operated 19 flights in January 2011 in which it used deicing fluid heated to temperatures that could damage planes.

Neither Southwest nor Great Lakes immediately responded to requests for comment. Both companies have 30 days to respond to the FAA.